The annual number of people becoming blindness in Japan reaches 16,000 and the causes thereof are broadly divided into two categories: the damage of the inner retinal layer and the damage of the outer retinal layer thereof, excluding the severe injury thereof. The outer retinal layer is known to be selectively degenerated in diseases such as retinitis pigmentosa and age-related macular degeneration.
Taking a look at the light reception of the retina and the signaling mechanism therein, the entered light and image information are received by photoreceptor cells (visual cells) located in the outer retinal layer, and transferred to the inner retinal layer. The information is transferred to the brain through ganglion cells constituting the optic nerve, located in the inner retinal layer. The cells thus capable of receiving light, or an image, are only visual cells in the retina; the degeneration or disappearance of visual cells for any cause will result in the loss of vision even when the other cells are normal.
According to a phototransduction pathway of visual cells, a chain reaction of various proteins is known to be necessary after the reception of light by a light-receiving protein present in the visual cells. Due to such a pathway, it has conventionally been considered that the introduction of a single gene cannot impart the ability to receive light. In this connection, it is reported that the impartment of the ability to receive light to nerve cells requires the introduction of at least 3 proteins (i.e., arrestin, opsin, and G-protein α-subunit) into one cell (Non Patent Literature 1).
Nagel et al. (Non Patent Literature 2) reported that channel rhodopsin-2 (hereinafter also referred to as “ChR2”) isolated from green alga Chamydomonas having the ability to receive light and cation-selective permeability and that the introduction of ChR2 gene could give the ability to receive light to cultured mammalian cells (HEK293 and BHK) and the like.
The present inventors reported that when rats with genetic blindness due to the damage of the outer retinal layer were attempted to be subjected to the introduction of channel rhodopsin-2 gene into the remaining neural retina cells to reconstruct visual function, no response (visual evoked potential) of the visual cortex to light was observed before the gene introduction, while the response thereof to light was observed after the gene introduction, and that the lowest illuminance at which the response of the visual cortex was observed was 240 lux in the rats, showing that their retina had a low light sensitivity compared to the normal retina (Non Patent Literature 3).
Thus, the light-receiving protein found in green algae is expected to be applied for vision. However, Chamydomonas-derived ChR2 has light sensitivity limited to a blue color; thus, only blue color can be seen even when vision has been restored.
A similar gene having sensitivity to a red color was found in green alga Volvox (Non Patent Literature 4); however, it is poor in the expression efficiency on a cell membrane and functionally insufficient when expressed in mammalian cells.